Ternary photoinitiator system for addition polymerization

ABSTRACT

Photopolymerizable compositions comprising: 
     (1) at least one vinyl monomr capable of undergoing free radical polymerization, 
     (2) a visible light or ultraviolet light responsive photoinitiator system comprising: 
     (a) a diaryliodonium salt, 
     (b) a pigment, insoluble in the photopolymerizable composition, said pigment also serving as a sensitizer, 
     (c) one or more electron donating compounds, and, optionally, 
     (3) adjuvants. 
     The three component photoinitiator system of this invention can increase, by several orders of magnitude, the rate of ultraviolet light or visible light photocuring of vinyl monomers.

BACKGROUND OF THE INVENTION

1. Technical Field

This invention relates to photoinitiator systems for use in addition,i.e., free radically-initiated, polymerization.

2. Discussion of Prior Art

Addition polymerization (also referred to as free radicalpolymerization) of vinyl monomers by exposure thereof to radiation fromthe ultraviolet or visible portion of the electromagnetic spectrum iswell known in the art. Polymerization of this type is also known asphotopolymerization, photocuring, or curing.

Aryliodonium salts have been previously described for use asphotoinitiators in addition-polymerizable compositions. Referencesrelating to such compositions include U.S. Pat. Nos. 3,729,313,3,741,769, 3,808,006, 4,228,232, 4,250,053 and 4,428,807; H. J. Timpeand H. Baumann, Wiss. Z. Tech. Hochsch. Leuna-Merseburg, 26, 439 (1984);H. Baumann, B. Strehmel, H. J. Timpe and U. Lammel, J. Prakt. Chem, 326(3), 415 (1984); and H. Baumann, U. Oertel and H. J. Timpe, Euro. Polym.J., 22, 313 (1986).

Photopolymerizable compositions usually contain a photoinitiator system,a polymerizable monomer, and optionally adjuvants. Often pigments areadded to color or opacify the compositions. The addition of pigmentsusually inhibits the initiation of polymerization because of competitivelight absorption or light scattering, which decreases the amount oflight available for absorption by the photoinitiator system. It would bedesirable to be able to use pigments as part of a photoinitiator systemin photopolymerizable composition. In the present invention, insolublesensitizers can serve both as sensitizers and colorants.

A three component system describing the use of iodonium salts, electrondonor compound, and soluble sensitizer is described in U.S. Ser. Nos.34,065 and 34,066, now U.S. Pat. No. 4,735,632 both filed Apr. 2, 1987.

However, these applications do not suggest the use of an insolublepigment as a sensitizer.

U.S. Pat. No. 4,257,915 discloses semiconductors in combination with anoxidation-reduction system for producing images. This patent does notteach the use of iodonium salts or organic pigments.

SUMMARY OF THE INVENTION

This invention provides compositions that are photopolymerizable by bothvisible light and ultraviolet (UV) light. More specifically, thisinvention provides photopolymerizable compositions comprising:

(1) at least one vinyl monomer capable of undergoing free radicalpolymerization,

(2) a visible light or ultraviolet light responsive photoinitiatorsystem comprising:

(a) a diaryliodonium salt,

(b) an organic or inorganic pigment, insoluble in the photopolymerizablecomposition, said pigment also serving as a sensitizer,

(c) one or more electron donating compounds selected from the group ofcompounds that possess a potential (E_(ox), donor) between about 0.5 and1.5 volts versus a saturated calomel electrode ("S.C.E."), and,optionally,

(3) adjuvants.

This invention also provides methods for the preparation ofphotopolymerized compositions.

Compositions of the present invention provide a very useful combinationof cure speed, cure depth, and shelf life. They can be used in a varietyof applications, including graphic arts imaging (e.g., for colorproofing systems, curable inks, or silverless imaging), printing plates(e.g., projection plates or laser plates), photoresists, solder masks,coated abrasives, magnetic media, photocurable adhesives (e.g., fororthodontics), and photocurable composites (e.g., for automobile repairor dentistry e.g., dental restoratives).

DETAILED DESCRIPTION OF THE INVENTION

Light responsive photoinitiator systems for initiation of thepolymerization of vinyl monomers comprise an inorganic or organicpigment, an electron donating, or donor, compound, and an electronaccepting. or acceptor, compound, such as, for example, onium salts. Awide variety of monomers can be photopolymerized using thephotoinitiator system of this invention. Suitable monomers for thisinvention contain at least one ethylenicallyunsaturated double bond andare capable of undergoing addition polymerization. These monomers canalso be oligomers. Suitable monomers include (1) mono-, di- or poly-acrylates and methacrylates, such as, for example, methyl acrylate,methyl methacrylate, ethyl acrylate, isopropyl methacrylate, n-hexylacrylate, stearyl acrylate, allyl acrylate, glycerol diacrylate,glycerol triacrylate, ethyleneglycol diacrylate, diethyleneglycoldiacrylate, triethyleneglycol dimethacrylate, 1,3-propanedioldiacrylate, 1,3-propanediol dimethacrylate, trimethylolpropanetriacrylate, 1,2,4-butanetriol trimethacrylate, 1,4-cyclohexanedioldiacrylate, pentaerythritol triacrylate, pentaerythritol tetraacrylate,pentaerythritol tetramethacrylate, sorbitol hexacrylate,bis[1-(2acryloxy)]-p-ethoxyphenyldimethylmethane,bis[1-(3-acryloxy-2-hydroxy)]-p-propoxyphenyldimethylmethane,trishydroxyethyl-isocyanurate trimethacrylate; (2) bis-acrylates andbis-methacrylates of polyethylene glycols of molecular weight 200-500,copolymerizable mixtures of acrylated monomers such as those describedin U.S. Pat. No. 4,652,274, and acrylated oligomers such as thosedescribed in U.S. Pat. No. 4,642,126; (3) unsaturated amides such as,for example, acrylamide, methylene bis-acrylamide, methylenebis-methacrylamide, 1,6-hexamethylene bis-acrylamide, diethylenetriamine tris-acrylamide, and beta-methacrylaminoethyl methacrylate; and(4) vinyl compounds such as, for example, styrene, diallyl phthalate,divinyl succinate, divinyl adipate, and divinylphthalate. Mixtures oftwo or more monomers can be used if desired.

The monomer is combined with a three component or ternary photoinitiatorsystem. The first component in the photoinitiator system is adiaryliodonium salt. The diaryliodonium salt should be soluble in thephotopolymerizable composition and preferably is shelf-stable (i.e.,does not spontaneously promote polymerization) when dissolved therein inthe presence of the pigment sensitizer and donor. Accordingly, selectionof a particular diaryliodonium salt depends, to some extent, upon theparticular monomer, pigment sensitizer and donor chosen. Diaryliodoniumsalts suitable for the present invention are described in U.S. Pat. Nos.3,729,313, 3,741,769, 3,808,006, 4,250,053 and 4,394,403, thedisclosures of which relating to aryliodonium salts are incorporatedherein by reference. The diaryliodonium salt can be a simple salt (e.g.,containing an anion such as Cl⁻, Br⁻, I⁻ or C₆ H₅ SO₃ ⁻) or a metal ormetalloid complex salt (e.g., containing an anion such as BF₄ ⁻, PF₆ ⁻,SbF₆ ⁻, SbF₅ OH⁻ or AsF₆ ⁻). Mixtures of diaryliodonium salts can beused if desired.

Preferred diaryliodonium salts include diphenyliodonium salts such asdiphenyliodonium chloride, diphenyliodonium hexafluorophosphate, anddiphenyliodonium tetrafluoroborate.

The second component in the photoinitiator system is the pigmentsensitizer. The pigment sensitizer can be an organic or inorganicmaterial that:

(1) is generally not soluble (solubility less than about 0.1 percent) inthe photopolymerizable composition,

(2) absorbs energy in a certain portion of the electromagnetic spectrum(for visible light, absorption is usually from about 380 to about 750nanometers (nm) and for ultraviolet light, absorption is usually fromabout 300 to about 380 nm), i.e. displays a band gap energy of about 1.5to about 4.0 electron volts, and

(3) behaves as a semiconductor, i.e., is capable of undergoingphotoredox reactions.

In conventional photosensitive compositions, pigments are usually viewedas a hindrance, because, by their nature, they competitively absorblight. Surprisingly, it has been found that insoluble organic andinorganic pigments actually serve as visible light and ultraviolet lightphotosensitizers for the light initiated polymerization of vinylmonomers when they are in combination with an aryliodonium salt and anelectron donor compound. Thus, certain pigments can actually replacesoluble dyes, which are normally used as sensitizers. It is preferredthat the pigment be dispersible with at least one of the othercomponents of the photopolymerizable composition.

Inorganic pigments suitable for use as sensitizers in this invention aredescribed in U.S. Pat. No. 4,257,915 and include, for example, titaniumdioxide, strontium titanate, barium titanate, zinc oxide, zinc sulfide,zinc selenide, cadmium sulfide, cadmium selenide, cadmium telluride.Organic pigments suitable for use as sensitizers in this inventioninclude, for example, phthalocyanine blue (pigment blue 15), copperpolychlorophthalocyanine green (pigment green 7), copperpolybromochlorophthalocyanines (pigment green 36), perylene scarlet (vatred 29), perylene vermillion (pigment red 123), perylene maroon,perylene bordeaux, perylene dianhydride (perylene red), and others suchas those described in "Pigments-Inorganic" and "Pigments-Organic" inKirk-Othmer Encyclopedia of Chemical Technology, Third ed., Volume 17,John Wiley and Sons (New York: 1982) pp. 788-871, semiconductingpolymers (see Ya. M. Paushkin, et al., Organic Polymeric Semiconductors,John Wiley & Sons, (New York: 1974), and photoconducting polymers (suchas described in J. M. Pearson, Pure and Appl. Chem., 49, 463-477(1977)).

The third component in the photoinitiator system is the electron donor.A wide variety of donors can be employed. The donor should not only besoluble in the photopolymerizable composition, but should also meet theoxidation potential (E_(ox)) requirements discussed in more detailbelow. Preferably, the donor is also selected on the basis of shelfstability. Accordingly, selection of a particular donor depends, inpart, on the particular monomer, diaryliodonium salt, and pigmentchosen. Donors are capable of increasing the speed of cure or depth ofcure of a composition of the invention upon exposure to light of thedesired wavelength. Preferably, E_(ox) of the donor is between about 0.5and 1.5 volts versus a saturated calomel electrode ("S.C.E."). Values ofE_(ox) of the donor can be measured experimentally, or obtained fromreferences such as N.L. Weinburg, Ed., Technique of ElectroorganicSynthesis Part II Techniques of Chemistry, Vol. V (1975), and C. K. Mannand K. K. Barnes, Electrochemical Reactions in Nonaqueous Systems(1970).

Preferred donors include amines (including aminoaldehydes andaminosilanes), amides (including phosphoramides), ethers (includingthioethers), ureas (including thioureas), ferrocene and its derivatives,sulfinic acids and their salts, salts of ferrocyanide, ascorbic acid andits salts, dithiocarbamic acid and its salts, salts of xanthates, saltsof ethylene diamine tetraacetic acid, and salts of tetralkyl andtetraaryl borates. The donor can be unsubstituted or substituted withone or more substituents that do not adversely affect thephotopolymerization reaction.

Preferred amine donor compounds include alkyl-, aryl-, alkaryl- andaralkyl-amines, such as, for example, methylamine, ethylamine,propylamine, butylamine, triethanolamine, amylamine, hexylamine,2,4-dimethylaniline, 2,3-dimethylaniline, o-, m- and p-toluidine,benzylamine, aminopyridine, N,N'-dimethylethylenediamine,N,N'-diethylethylenediamine, N,N'-dibenzylethylenediamine,N,N'-diethyl-1,3-propanediamine, N,N'-diethyl-2-butene-1,4- diamine,N,N'-dimethyl-1,6hexanediamine, piperazine,4,4'-trimethylenedipiperidine, 4,4,-ethylenedipiperidine,p-N,N-dimethylaminophenethanol and p-N,N-dimethylamino-benzonitrile;aminoaldehydes, such as, for example, p-N,N-dimethylaminobenzaldehyde,p-N,Ndiethylaminobenzaldehyde, 9-julolidine carboxaldehyde, and4-morpholinobenzaldehyde; and aminosilanes, such as, for example,trimethylsilylmorpholine, trimethylsilylpiperdine,bis(dimethylamino)diphenylsilane, tris(dimethylamino)methylsilane,N,N-diethylaminotrimethy1silane, tris(dimethylamino)phenylsilane,tris(methylsilyl)amine, tris(dimethylsilyl)amine,bis(dimethylsilyl)amine, N,N-bis(dimethylsilyl)aniline,N-phenyl-N-dimethylsilylaniline and N,N-dimethyl-N-dimethylsilylamine.Tertiary aromatic alkylamines, particularly those having at least oneelectron-withdrawing group on the aromatic ring, have been found toprovide especially good shelf stability.

Preferred amide donor compounds include N,N-dimethylacetamide,N,N-diethylacetamide, N-methyl-N-phenylacetamide,hexamethylphosphoramide, hexaethylphosphoramide,hexapropylphosphoramide, trimorpholinophosphine oxide, andtripiperidinophosphine oxide.

Suitable ether donor compounds include 4,4'-dimethoxybiphenyl,1,2,4-trimethoxybenzene, and 1,2,4,5-tetramethoxybenzene.

Suitable urea donor compounds include N,N'-dimethylurea,N,N-dimethylurea, N,N'-diphenylurea, tetramethylthiourea,tetraethylthiourea, tetra-n-butylthiourea, N,N-di-n-butylthiourea,N,N'-di-n-butylthiourea, N,N-diphenylthiourea, andN,N'-diphenyl-N,N'-diethylthiourea.

The three necessary components of the composition of this invention arepresent in "photochemically effective amounts", that is, the amount ofeach component must be sufficient to enable the monomer to undergophotochemical gelation or hardening upon exposure to light of thedesired wavelength. Amounts of each component are independently variableand thus need not be equal, with larger amounts of diaryliodonium saltand/or electron donor compound generally providing faster cure, butshorter shelf life. Use of too much pigment or improper preparation ofthe dispersion can adversely affect the cure.

Compositions of this invention can contain a wide variety of adjuvantsdepending upon the desired end use. Classes of adjuvants suitable forthis invention include solvents, diluents, resins, binders,plasticizers, dyes, inorganic or organic reinforcing or extendingfillers, thixotropic agents, indicators, surfactants, inhibitors,stabilizers, UV absorbers, dispersing aids, and the like. An additionalcomponent that can be added to the composition of this invention is aradiation-sensitive dye. Suitable dyes include those that sensitize thephotoconductivity of semiconductor pigments, particularly those thatabsorb visible radiation. Such dyes are discussed by Meier in SpectralSensitization, Focal Press Limited, London, 1968, pp. 101 to 110.Examples of such suitable dyes include Acriflavine CI 46000, Rose BengalCI 45435, Acridine Orange CI 46005, Kryptocyanine, Fuchsine, MalachiteGreen, Erythrosine, Eosine, Fluorescein, Phthalocyanine, and merocyaninedyes as taught in U.S. Pat. No. 4,257,915. Amounts and types of suchadjuvants, and their manner of addition to a composition of theinvention will be familiar to those skilled in the art.

Suitable ratios of ingredients include the following: for every 100parts by weight of the photopolymerizable composition there may bepresent from 1 to 99 parts by weight vinyl monomer, from 0.001 to 50parts by weight pigment sensitizer, from 0.005 to 10 parts by weight ofdiaryliodonium salt, from 0.005 to 10 parts by weight of donor compound,from 0 to 99 parts by weight of adjuvants. Preferably there is used per100 parts by weight of the photopolymerizable composition from 5 to 60parts by weight vinyl monomer, 0.1 to 30 parts by weight of pigmentsensitizer, 0.1 to 5 parts by weight of diaryliodonium salt, 0.1 to 5parts by weight of donor compound, and from 5 to 90 parts by weight ofadjuvants.

In making the photosensitive compositions of this invention, the pigmentsensitizer, diaryliodonium salt, donor compound, polymerizable monomeror monomers, and optionally adjuvants can be combined in any order andmixed until a homogeneous dispersion or mixture is obtained. Fillers andother adjuvants can be added as desired. It has been found desirable tostore the compositions of this invention away from light until it isdesired to initiate polymerization by the intentional exposure to light.

Compositions of this invention can be cured by means of a variety ofmethods. It is convenient to expose the compositions to actinicradiation by employing light sources that emit ultraviolet or visiblelight, such as, for example, quartz halogen lamps, tungsten-halogenlamps, mercury arcs, carbon arcs, low-, medium-, and high-pressuremercury lamps, plasma arcs, light emitting diodes and lasers. Electronbeam ("E-beam") irradiation and other curing devices that do not dependon light emission can also be employed. In general, heat applied duringor after exposure and/or an inert atmosphere will accelerate cure.

Insoluble sensitizer pigments are introduced as dispersions in place ofthe usual soluble organic sensitizers. Dispersions can be prepared bydispersing the pigment in water. While not necessary, surfactants andwater soluble polyvinylalcohol (PVA) can be used as additives in thepreparation of the dispersions. Stock dispersions can be prepared bymeans of a ball mill. A photoinitiator system comprising a mixture ofthis dispersion, an electron donor compound, and a diaryliodonium salt,can be used to polymerize a vinyl monomer by means of exposure tovisible or ultraviolet light.

The three component photoinitiator system of this invention hasincreased by several orders of magnitude the rate of ultraviolet lightor visible light photocuring of vinyl monomers.

This invention is further illustrated by the following examples, but theparticular materials and amounts thereof recited in these examples, aswell as other conditions and details, should not be construed to undulylimit this invention to the illustrative embodiments set forth herein.

EXAMPLES

In the following examples, parts are reported as parts by weight and allpercentages are given as percentages by weight unless indicatedotherwise. All temperatures are reported in degrees Centigrade andexamples were conducted at ambient temperature (about 20-25° C.) unlessindicated otherwise. Monomers, diaryliodonium salts, pigments, electrondonor compounds, adjuvants, resins, and the like used in the exampleswere commercially available unless indicated otherwise.

Light used to initiate the polymerizations was from a visible lightsource unless indicated otherwise.

EXAMPLES 1-4

The following example describes the use of pigments as sensitizers toinitiate free radical polymerization of vinyl monomers using the threecomponent photoinitiator system, i.e., pigment, diaryliodonium salt, andelectron donor compound.

Pigments used in this example were prepared as water dispersionsaccording to the following procedure: a solution containingpolyvinylalcohol (PVA) (12 g), cetyltrimethylammonium bromide surfactant(0.2 g, Aldrich Chemical Co., Milwaukee, WI) and water (200 g) wasprepared. Two different polyvinyl alcohols, both of which arecommercially available from Aldrich Chemical Co., Milwaukee, WI, wereused; one had a molecular weight of 2,000 and was 75% hydrolyzed, andthe other had a molecular weight of 10,000 and was 88% hydrolyzed. Tothe solution was added 5 g of the desired pigment. The resultant mixturewas placed on a ball mill and milled until a satisfactory stockdispersion was obtained as indicated by visual examination of thedispersion spread on a glass slide through a 100 X microscope. Fineparticles indicated a good, suitable dispersion.

For each sample, a dilution of 1/19, volume per volume, dispersion/waterwas made. A 4 ml portion of this diluted mixture was placed in a "Pyrex"test tube, and acrylamide (about 0.3 g) was added thereto. Depending onthe sample, either sodium p-toluenesulfinate (about 0.03 g) ordiphenyliodonium chloride (about 0.03 g) or both were added. Each samplewas purged with nitrogen for two minutes before and continuously duringa five minute exposure to a light source ("Kodak Carousel" projector,500 watts) using a 400 nm filter between the source and sample. Eachsample was then poured intomethanol (30 ml) to cause precipitation ofany polymer. The following table shows polymerization results ofcompositions of the present invention, along with comparative results.In the table, column A shows results for compositions containing pigmentsodium p-toluenesulfinate, column B shown results for compositionscontaining pigment plus diphenyliodonium chloride, and column C showsresults for compositions containing pigment, sodium p-toluenesulfinate,and diphenyliodonium chloride.

In the table, N means no polymer was detected, S means a small amount ofpolymer was formed, and P means a large amount of polymer was formed.

                  TABLE I                                                         ______________________________________                                        Example   Pigment            A      B    C                                    ______________________________________                                        1         Phthalocyanine Blue.sup.1                                                                        N      N    P                                    2         Perylene Red.sup.1 S      N    P                                    3         Cadmium Sulfide.sup.2                                                                            S      N    P                                    4         Poly(p-aminobenzaldehyde).sup.2                                                                  S      N    P                                    ______________________________________                                         .sup.1 PVA, molecular weight 2000, 75% hydrolyzed, 0.2 g surfactant           ("Triton X100") added to dispersion.                                          .sup.2 PVA, molecular weight 10000, 88% hydrolyzed.                      

Only the samples containing pigment underwent decided polymerization.

EXAMPLES 5-8

The following example describes a coating formulation comprising amixture of vinyl monomers, a diaryliodonium salt, an electron donorcompound, and a pigment as a sensitizer and a process wherein themixture is photopolymerized to provide a coated film. The sensitizerpigments of the examples are indicated in Table 2 below. The exampleshows the increase in speed provided by photoinitiator systemscontaining pigment sensitizer, diaryliodonium salt, electron donorcompound (ternary systems) as compared to photoinitiator systemscontaining pigment sensitizer-diarylidonium salt or pigmentsensitizer-electron donor compound (binary systems) for the initiationof free radical polymerization.

Stock coating formulations were prepared by using a suitable stockdispersion of the pigment prepared according to the process set forth inExamples 1-4 and adding to it 12 g of acrylamide and 1 g ofN,N'-methylenebisacrylamide, followed by ball milling until mixing wascomplete. A small amount of surfactant, about 0.05 g of "Triton X-100"surfactant or "Nelkal NF" surfactant (GAF Corporation, New York, NY),was added to improve the coating characteristics of these formulations.

Coating samples were prepared by taking 25 g of the above stock coatingformulation and adding 0.1 g of the appropriate electron donor compoundand/or diaryliodonium salt. Samples were coated onto a substrate ofgelatin-subbed 3 mil polyester film (3M Co., St. Paul, MN) using anumber eighteen wire wound rod. The coated film was dried first by aheat gun and then in an oven at 60° C. for three minutes. Allmanipulations using visible light sensitive materials were performedunder red safelights and UV sensitive materials were correspondinglyhandled under yellow safelights. Visible light exposures were made witha Model 70 tungsten light source (3M Co., St. Paul, MN) in a commercialvacuum frame and UV exposures were made with a 2 kilowatt Berkey Ascor(Berkey Technical Company, Woodside, NY) light source with photopolymerbulb in a commercial vacuum frame.

The resultant exposed, coated films were then washed with awater/methanol solvent mixture for development to remove unpolymerizedmaterial. Polymerization in a coated film was detected by exposing afilm in an imagewise manner and revealing the retained image by washingof the film to remove unpolymerized materials. Relative speed ofpolymerization was determined by the number of steps of a Stouffer 21step wedge (Stouffer Graphic Arts, South Bend, IN) remaining afterwashing. In Table 2 column A shows results for compositions containingpigment plus sodium p-toluenesulfinate, column B shown results forcompositions containing pigment plus diphenyliodonium chloride, andcolumn C shows results for compositions containing pigment, sodiump-toluenesulfinate, and diphenyliodonium chloride.

                  TABLE 2                                                         ______________________________________                                                                   Number                                                                 Light  of steps                                           Example Pigment           source   A   B   C                                  ______________________________________                                        5       Phthalocyanine Blue.sup.1,5                                                                     Visible  N   L   8                                  6       Perylene Red.sup.1,4,5                                                                          Visible* N   L   14                                 7       Poly              Visible* N   N   10                                         (p-aminobenzaldehyde).sup.3,4 6                                       8       Cadium Sulfide.sup.3,4,5                                                                        UV       L   L   13                                 9       Zinc Oxide.sup.2,3,6                                                                            UV       L   L   7                                  10      Titanium Dioxide.sup.2,3,6                                                                      UV       L   L   8                                  ______________________________________                                         *10 second light exposure                                                     .sup.1 PVA, molecular weight 2000, 75% hydrolyzed.                            .sup.2 PVA, molecular weight 10000, 88% hydrolyzed.                           .sup.3 surfactant ("Triton X100") added.                                      .sup.4 surfactant ("NekalNF") added.                                          .sup.5 3/20, V/V, methanol/water used for development.                        .sup.6 3/10, V/V, methanol/water used for development.                   

Results are shown in Table 2 where N means that no image was formed andL means that an image was formed but was lost upon development. Whereexposures were made of coated films without the added diaryliodoniumsalt/electron donor compound, no image was detected except in the caseof cadmium sulfide. This image was lost on washing. Coatings containingonly the diaryliodonium salt/electron donor compound combination did notimage under these conditions.

What is claimed is:
 1. A photopolymerizable composition comprising:(a)at least one vinyl monomer capable of undergoing free-radicalpolymerization, and (b) a photoinitiator system comprisingphotochemically effective amounts of(i) a diaryliodonium salt, (ii) anorganic or inorganic pigment, insoluble in the photopolymerizablecomposition, which serves as a sensitizer, and (iii) at least oneelectron donor compound, said donor being different from said pigmentand having a potential between about 0.5 and 1.5 volts versus asaturated calomel electrode.
 2. A composition according to claim 1,wherein said diaryliodonium salt comprises a diphenyliodonium simplesalt or diphenyliodonium metal complex salt.
 3. A composition accordingto claim 1, wherein said sensitizer pigment is selected from the groupconsisting of phthalocyanines, porphyrins, and perylenes.
 4. Acomposition according to claim 1, wherein said sensitizer pigment is aphthalocyanine.
 5. A composition according to claim 1, wherein saidsensitizer pigment is a porphyrin.
 6. A composition according to claim1, wherein said sensitizer pigment is a perylene.
 7. A compositionaccording to claim 1, wherein said sensitizer pigment is selected fromthe group consisting of cadmium sulfide, cadmium selenide, cadmiumtelluride, zinc oxide, zinc sulfide, and titanium dioxide.
 8. Acomposition according to claim 1, wherein said E_(ox) (donor) is betweenabout 0.5 and 1 volt versus a saturated calomel electrode.
 9. Acomposition according to claim 1, wherein said donor is selected fromthe group consisting of amines, amides, ethers, ureas, ferrocenederivatives, sulfinic acids and their salts, ferrocyanide salts,ascorbic acid and its salts, dithiocarbamic acid and its salts, xanthatesalts, ethylene diamine tetraacetic acid salts, tetraalkyl borate salts,tetraaryl borate salts.
 10. A composition according to claim 1, whereinsaid donor comprises sulfinic acid salts.
 11. A composition according toclaim 10, wherein said donor comprises a member of the group selectedfrom tetraalkyl and tetraaryl borate salts.
 12. A composition accordingto claim 1, wherein said composition contains, for every 100 parts byweight of composition, about 1 to 99 parts by weight of said monomer,about 0.005 to about 10 parts by weight of said diaryliodonium salt,about 0.001 to about 50 parts by weight of said sensitizing pigment, andabout 0.005 to about 10 parts by weight of said donor.
 13. A compositionaccording to claim 12, wherein said composition contains, for every 100parts by weight of said composition, about 5 to 60 parts by weight ofsaid monomer, about 0.1 to about 5 parts by weight each of saiddiaryliodonium salt, about 0.1 to about 3 parts by weight of saidsensitizing pigment, and about 0.1 to about 5 parts by weight of saiddonor.
 14. A composition according to claim 12, wherein said compositionfurther comprises about 10% to 90% by weight of fillers, based on thetotal weight of said composition.
 15. A composition according to claim1, further including adjuvants.
 16. An article comprising a supportbearing a coating comprising the composition of claim
 1. 17. An articleaccording to claim 16 wherein said pigment is selected from the groupconsisting of phthalocyanines, perylenes, and cadmium sulfide.